NEW YORK (Reuters) - Bitcoin fell to a 1-1/2 week low on Wednesday after the U.S. Securities and Exchange Commission warned of "potentially unlawful" systems that trade crypto-currencies, which investors may use with an unearned sense of safety.

At 1:07 p.m. (1807 GMT), the world's biggest and best-known digital currency <BTC=BTSP> was last down 7.53 percent at $9,924.03 on the Luxembourg-based Bitstamp exchange. It fell as low as $9,450 earlier Wednesday, which was the lowest since Feb. 26.